i fitted a firetec foam in the engine bay and carry a 3 litre foam in the van between the seats, the reason i haven't used powder is because it's very corrosive and messy if you need to use it - foam will just rinse off should you need to, whatever you do don't carry 2 types and mix the media if you need to use them they do act differently, foam or powder is my choice - co2 seems pointless on fuel fires where petrol could be feeding the flame, it mainly works by removing the oxygen and you need something to smother the fire and cool it so foam suits that well, there's always going to be someone who disagrees but that's my take on it.